In general, oil, gas, water, geothermal or analogous wells, which are more than a few hundreds of meters deep, contain a steel lining called the casing. The annular space between the underground formation and the casing is cemented over all or a large portion of its depth. The essential function of the cement sheath is to prevent fluid migration along the annulus and between the different formation layers through which the borehole passes and to control the ingress of fluid into the well.
However, this zonal isolation may be lost for a number of reasons. Mud may remain at the interface between the cement and the casing and/or the formation. This forms a path of least resistance for gas or other fluids movement. Changes in downhole conditions may induce stresses that compromise the integrity of the cement sheath. Tectonic stresses and large increases in wellbore pressure or temperature may crack the sheath and may even reduce it to rubble. Radial displacement of casing, caused by cement bulk shrinkage or temperature decreases, as well as decreases in fluid weight during drilling and completion, may cause the cement to debond from the casing and create a microannulus. Routine well-completion operations, including perforating and hydraulic fracturing, negatively impact the cement sheath.
Various methods are used to attempt to prevent a film of mud forming on the casing/formation surface. The most common methods involve use of spacers and wash fluids to remove as much as possible of the remaining mud and the mud filter cake from the walls of the wellbore. This process has been the subject of continuous modification and improvement over the past several decades, but success has been limited by the operational conditions and the limited amount of time and resources that can be put into these operations. As a result, the efficiency of mud removal is often less than desired.
On the other side, mechanical properties of cement, such as elasticity, expandability, compressive strength, durability and impact resistance have been improved, in particular, by the addition of fibres and/or plastic or metallic particles. Increased flexibility helps the cement respond to thermal, mechanical or pressure shocks and can minimize debonding of the cement from the metal casing or from the formation wall. Fibres are best at handling mechanical shocks, such as those encountered when one needs to drill through an existing cement sheath in order to form a lateral arm of the well. This is an important part of the construction of multilateral wellbores. Expandability ensures that the cement is held in compression behind casing thus allowing for pressure drops in the annulus without debonding between the casing and the isolating material. In this case, the expansion needs to be tailored to the mechanical properties of the formation and to the cement in order to be effective. These properties are not always known in sufficient detail to achieve optimal performance.
Also, various methods have been proposed to improve the sealing of the formations, including the use of cement with additives such as silicone as described in the U.S. Pat. No. 6,196,316 or epoxy resin (e.g. U.S. Pat. No. 6,350,309). In U.S. Pat. No. 5,992,522 the hydrostatic pressure of a column of bitumen is used to prevent vertical migration of fluids in a wellbore.
Other completion techniques are so-called “open hole” completions as often encountered in laterally extended wells. In open hole completion, the casing or production tubing is not cemented and zonal isolation when required is achieved by using packers. Packers are constituted by annular sealing rings comprising a double elastomer wall reinforced with a metal braid. The double wall delimits a chamber, which is usually inflated by cement or other suitable compositions such as expanding resin (as described in U.S. Pat. No. 5,190,109). Packers suffer from limitations and drawbacks, which are outlined, for example, in the U.S. Pat. No. 4,913,232 and are often not suitable for permanent wellbore installations.
Thus, there is a need for methods and systems that can be placed at key positions to provide zonal isolation or plugging in the wellbore. Further, there is a need for a single approach that can be used in a majority of completions. There is a need for a process that can be executed efficiently and reliably in the oilfield. There is further a need for a solution that, while generically useful, can readily be tailored to survive different down-hole environments such as maximum temperature and fluid exposure for an extended period of time, and ideally over the lifetime of the well. These fluids could be brines, hydrocarbons, carbon dioxide, hydrogen sulphide and may further include aggressive treatment fluids such as hydrochloric acid.